The present invention relates to a porous catalyst with controlled pore sizes comprising layered silicates and metal oxides and a preparation method thereof. The present invention also relates to a method for preparing xcex5-caprolactam using-the porous catalyst in a vapor phase.
The manufacture of xcex5-caprolactam from cyclohexanone oxime via Beckmann rearrangement reaction is an industrially important process. xcex5-Caprolactam is used as a raw material for nylon-6 synthesis. The commercial process for producing xcex5-caprolactam is now being carried out in a liquid phase using a fuming sulfuric acid as a catalyst. In such a process, it is necessary to neutralize the sulfuric acid with ammonia in order to separate the produced is lactam, whereby ammonium sulfate is produced as by-products of 2-3 tons per 1 ton of the xcex5-caprolactam produced. Thus, the recovery and disposal of the by-products add substantial and extra cost to the production of xcex5-caprolactam. In addition, the sulfuric acid causes corrosion of the reaction vessel, whereby safety and environmental problems in treating concentrated sulfuric acid are also posed.
To resolve these problems, various studies have been made to develop solid acid catalysts for the Beckmann rearrangement reaction in a vapor phase since mid of 1960""s. For example, boric acid (German Patent No. 10920), silica-alumina (UK Patent No. 881,927), phosphoric acid (UK Patent No. 881,276), zeolites (Journal of Catalyst, Vol. 6, p 247, 1966), tantalum oxide supported on silica (European Patent Publication No. 509,493 A1) have been proposed as solid acid catalysts for the Beckmann rearrangement reaction of cyclohexanone oxime.
European Patent Publication No. 509,493 A1 discloses a method for preparing xcex5-caprolactam, which comprises subjecting cyclohexanone oxime to a Beckmann rearrangement reaction in a vapor phase in the presence of tantalum oxide supported on silica, wherein the catalyst is prepared by contacting tantalum ailkoxide to a ammorphous silica. (Ta2O5 1-30 wt % /SiO2) In these cases, the pore structure of silica carriers was important for the catalytic performance. The high ratio ( greater than 90%) of the pore volume for fine pores with diameters from 40 to 2,000 xc3x85 to total pore volume (40 to 150,000xc3x85) of silica carriers was effective.
Japanese published unexamined patent application No.139062/1982 discloses crystalline alumino-silicates e.g. ZSM-5 having 40-60 of Si/Al atomic ratio show low selectivity of xcex5-caprolactam although conversion of cyclohexanone oxime is said to be nearly complete. Whereas U.S. Pat. No. 4,709,024 discloses the catalytic activity of ZSM-5 improves with the increase in the Si/Al ratio to 500. These studies indicate that high Si/Al ratio in MFI zeolites give an improved catalytic performance for the Beckmann rearrangement of cyclohexanone oxime.
U.S. Pat. No. 4,717,769 further discloses the production of xcex5-caprolactam by a vapor-phase catalytic rearrangement of the cyclohexanone oxime in the presence of crystalline metallo-silicates. These include Ga, Fe, B, Zr, Bi, Nb, Zn, Be, Cr, La, Ti, Hf, V and/or Cu substituted forms in high Si/Al ratio of MFI zeolites. Although the improved solid acid catalysts show a high conversion for the cyclohexanone oxime and selectivity for the caprolactam at initial stage, the catalytic activities do not last for a long period. To replace homogeneous process with vapor phase process in the industrial mass production of xcex5-caprolactam, the solid acid catalyst must have an improved catalytic activity and it""s lifetime;
According to the many studies reported, the solid acid catalyst should have weak acidic hydroxyl groups attached preferably on the surface of the solid acid catalysts for Beckmann the rearrangement of cyclohexanone oxime. It is suggestive that an efficient solid acid catalyst for the rearrangement of cyclohexanone oxime can be achieved if the catalyst possesses abundant hydroxyl groups having weak acidity. Layered silicates, such as kanemite (NaHSi2O53H2O), magadiite (Na2Si14O2911H2O), kenyaite (K2Si20O411H2O makatite (Na2Si4O95H2O) and ilerite (Na2Si8O17xH2O) are composed of tetrahedral silicate sheets only and the each silicate sheet terminates with the hydroxyl groups. The layered silicates, therefore, possess abundant hydroxyl groups, and have a great potential to act as new catalysts for the vapor phase Beckmann rearrangement reaction.
In the present invention, an efficient solid acid catalyst for the vapor phase Beckmann rearrangement reaction is designed using layered silicates Small amount of metal oxide was intercalated between the silicate layers by usual pillaring process to generate moderate acidity as a mixed oxide and to increase thermal stability of layered silicates for use in catalysis.
The present invention provides a new porous catalyst for preparing xcex5-caprolactam comprising layered silicates and metal oxides to solve the problems occurred in the conventional techniques.
Another object of the present invention is to provide a method for preparing the porous catalyst, wherein the method comprises the steps of substituting Na+ ion of the layered silicate with H+ ion in an inorganic acid solution, expanding the interlayer distance between the silicate layers to 20-30 xc3x85 using a long chain amine, inserting metal oxide in the above silicate layers and calcinating the resultant product.
The present invention relates to a porous catalyst comprising layered silicates and metal oxides whose pore sizes are controlled, a method for preparing the porous catalyst, and a method for preparing xcex5-Caprolactam from cyclohexanone oxime using the porous catalyst in a vapor phase.
The present inventors have succeeded in preparing weak solid acid catalyst by pillaring metal oxides to layered silicate that possesses abundant hydroxyl groups. The inventors also succeeded in using this solid acid catalyst as a catalyst for a Beckmann rearrangement of cyclohexanone oxime. The catalyst of the present invention has a higher conversion for cyclohexanone oxime, a high selectivity for xcex5-caprolactam and a long life time when compared to the zeolite catalyst having MFI structure with a high silicate composition.
The present invention provides a layered silicate catalyst pillared with metal oxide and a method for preparing the same. More particularly, the porous catalyst of the present invention comprises layered silicates and metal oxides. The metal oxides are pillared between the silicate layers in the present invention. The method for preparing a porous catalyst comprises the steps of substituting Na+ ion of the layered silicate with H+ ion in an inorganic acid solution, expanding the interlayer distance between the silicate to 20-30 xc3x85 using a long chain amine such as octylamine, inserting metal oxides between the silicate layers and calcinating the resultant product. The method for pillaring metal oxide between the layered materials is generally well known. However, it has not been used as catalyst for the Beckmann rearrangement of cyclohexanone oxime prior to the present invention.
In the present specification, xe2x80x9cpillaring of metal oxides between silicate layersxe2x80x9d means that metal oxides are inserted between silicate layers like rods such a manner that the pillared metal oxide is bonded to the two layers and thus the structure of the layered silicates is maintained
Layered silicates have a two-dimensional structure, and abundantly exist in the nature. Thus, the layered silicates are commercially available or some of the layered silicates can be easily synthesized in the laboratory. Unlike clays, the layered silicates comprise only tetrahedra of SiO4 and cation such as Na+. The structure of the layered silicates is maintained through weak electrostatic interactions between the negatively charged silicate layers and positively charged Na+ ion. The Na+ ion can be easily substituted with other ions through an ion exchange method. For instance, Na+ ions between the layers can be easily exchanged with H+ ions in inorganic acid solutions such as hydrochloric acid or sulfuric acid. Small amount of metal oxide was intercalated between the silicate layers by usual pillaring process to generate moderate acidity as a mixed oxide and increase the thermal stability of the layered material for use in catalysis. Silica layers pillared with metal oxide have a superior property as a solid acid catalyst. It has been reported that the layered silicates pillared with metal oxides such as Cr, Mo, W, Fe, Co and Ni were used as catalysts for hydrocarbon cracking reaction (EP 508005, JP 05-271669).
In the layered silicate catalysts of the present invention, metal oxide can be pillared between the layered silicates in the amount of 0.5-30 weight %, preferably, 2-10 weight %. The interlayer distance between the silicate layers is 5 to 30 xc3x85, preferably 5 to 15 xc3x85, and the surface area of the prepared catalyst is 100-500 m2/g, preferably 100-300 m2/g.
The layered silicate materials that can be used in the present invention include kanemite, magadiite, kenyaite, makatite, ilerite, and octosilicate. Some of these can be easily synthesized in the laboratory.
The catalyst of the present invention can be prepared by pillaring a variety of metal oxides such as Ta2O5, Nb2O5, TiO2, or SiO2 between the silicate layers. The catalyst have internal pores of about 5 to 30 xc3x85 in diameter.
When the layered slicate catalyst pillared with metal oxide used in Beckmann rearrangement of cyclohexanone oxime the catalytic activity at the initial stage of the reaction is high; the conversion of cyclohexanone oxime is 97% and the selectivity for caprolactam is 95%. In addition, the catalyst of the present invention shows high conversion and selectivity for a long period of time and is superior to that of Silicalite-1 (pure silica ZSM-5), which is known to be good solid acid catalyst for the Beckmann rearrangement of cyclohexanone oxime up to date.